Instead of conventional induction motors (IM motors), permanent magnet synchronous motors (PM motors), which have excellent efficiency and can be expected to be reduced in size and noise, have come to be widely used. For example, as drive motors for railway cars and electric cars, the PM motors have come to be utilized.
Since an IM motor generates a magnetic flux itself by an excitation current from a stator, it has a technical problem of causing a loss due to flowing of the excitation current.
On the other hand, since a PM motor a motor including a permanent magnet in its rotor to output a torque by utilizing its magnetic flux, it is free from such a problem in an IM motor. However, a PM motor generates an induced voltage (counter-electromotive voltage) corresponding to its rotational speed due to the permanent magnet. In a applied fields with its wide rotation range such as railway cars, automobiles and the like, it is necessary to satisfy a condition that its inverter for driving and controlling the PM motor is not damaged due to an induced voltage generated at the maximum rotational speed (an overvoltage). In order to satisfy this condition, it is necessary to increase voltage resistance of the inverter sufficiently, or, restrict, on the contrary, the magnetic flux of the permanent magnet included in the motor. Since the former action may affect its power supply, the latter action is often selected. When an amount of magnetic flux in such a case is compared with that of magnetic flux of an IM motor (an amount of gap magnetic flux generated by the excitation current in a case of an IM motor), their ratio may be about one to three. In this case, in order to generate the same amount of torque, it is necessary to supply a high (torque) current in a PM motor that generates a small amount of magnetic flux. Thus, when currents necessary to output the same amount of torque in a low speed range for an IM motor and a PM motor are compared, it is necessary to supply a higher current to a PM motor.
Hence, as compared with an IM motor, current capacity of an inverter for driving a PM motor increases. Moreover, in general, since switching frequency of a switching element included in the inverter is high at a low speed and its resulting loss increases depending its current, a large loss and heat will be generated at a low speed in a PM motor.
Since cooling owing to running wind is expected in respect to an electrical train, an inverter device is inevitably increased in size due to needs for enhancing its cooling performance in a case where a large loss might occur at a low speed. In contrast, although field weakening control might be performed in a case where its induced voltage is high, its efficiency is reduced due to superposition of excitation currents.
As described above, the PM motor has merits and demerits resulting from the inclusion of the magnet. As a motor, a PM motor has a larger number of merits, and this reduces its loss and size; on the other hand, when the PM motor is used in an electrical train, an electric car or the like in which variable speed control is performed, it has poor efficiency as compared with a conventional IM motor at some operating points. Moreover, since an inverter is increased in its current capacity and loss, the device is increased in size. With respect to system's efficiency itself, its overall efficiency is improved by use of a PM motor because a side of the motor is dominant; on the other hand, it is undesirable that the increase in the size of the inverter becomes the demerit of the system.
In a patent document 1, disclosed is an AC motor for driving an electric car that increases efficiency of a system by an operation of the motor and an inverter at a high efficiency both in a low power operation and a high power operation. In the AC motor for driving an electric car, magnetic flux generated by a permanent magnet embedded in a magnetic field pole and, as required, the magnetic flux generated by an excitation coil produce magnetic field flux, and, according to the output of the motor, a generating source of the magnetic field flux switches between only the permanent magnet and both the permanent magnet and the excitation coil, and an excitation current is supplied through a rotary transformer.
Hence, the AC motor for driving an electric car can switch to an operation with the permanent magnet alone according to the output of the motor at the time of, for example, a low output, and thus increases its operation efficiency. Furthermore, since a motor voltage can be increased in a low speed range of the motor, it is possible to reduce the current and thereby the efficiency of the system can be enhanced due to reduction of a loss in copper of the motor winding wire and a loss produced in the inverter. These effects are advantageous particularly to electric cars which are often operated in a low/medium speed range, and it can be made possible to enhance the efficiency at which the current is used and to extend a running distance achieved by a single charge.
Moreover, since the AC motor for driving an electric car does not demagnetize the permanent magnet, it is possible not only to simplify the inverter control but also to protect the device by preventing an abnormal overvoltage. The rotary transformer can be reduced in size by being operated at a high frequency, and thus it is possible to reduce the motor and the entire system in size and weight.    Patent document 1: Japanese Patent Application Laid-open No. H05-304752
FIG. 25 is a block diagram showing an example of a permanent magnet reluctance motor drive system. This system is configured with a smoothing capacitor 102, a DC power supply 3, an inverter 4 that converts a DC power to an AC power and a permanent magnet reluctance motor 1a that is driven by the AC power of the inverter 4. The inverter 4 converts the DC power from the DC power supply 3 to the AC power and supplies it to the permanent magnet reluctance motor 1a. 
The permanent magnet reluctance motor 1a operates at a higher efficiency than an induction motor, and is advantageously of a small size and a high output. Furthermore, the permanent magnet reluctance motor 1a is, since it can operate with its speed variable in a wide range, often used in an electric car or a hybrid car.
However, the efficiency of the permanent magnet reluctance motor varies depending on conditions for its rotational speed and torque. Thus, when the permanent magnet reluctance motor is used in a electrical train, an electric car, a hybrid car or the like, it does not always achieve optimum performance in all the operational range in terms of its torque and rotational speed, and some conditions are present on which the efficiency is poor.
Therefore, it is possible to utilize a variable magnetic flux drive system using a variable magnetic flux motor that can vary the magnetic flux of the magnet with the current of the inverter. Since this system can vary an amount of magnetic flux of the permanent magnet with a short-time magnetization current according to operational conditions, it can be expected that the efficiency is enhanced as compared with the conventional permanent magnet reluctance motor. Moreover, when the magnet is unnecessary, it is possible to minimize the induced voltage by reducing the amount of magnetic flux.
When the magnetization current is supplied through the variable magnetic flux motor, it is necessary to use a higher DC voltage that is input into the inverter as compared with a DC voltage used at the time of a normal operation. However, when a rotational speed of the variable magnetic flux motor is small, it is unnecessary to use the high voltage. Thus, when it is used in a device, such as an air conditioner, that can reduce the rotational speed for a short moment without problems, the magnetization is preferably performed at a reduced rotational speed. However, when the variable magnetic flux motor is used in a power source in an electrical train or an electric car, it is impossible to reduce the rotational speed each time of magnetizing, and this results in a problem.
Even when a high-voltage voltage source is initially used, a use of a secondary battery as the voltage source causes variations in the voltage at the time of charging and discharging, and thus the voltage required for the magnetization is not always provided reliably. Since a high voltage is needed only in a short time period of the magnetization, it is inefficient to initially use the high-voltage voltage source.
Any device or product usually has a plurality of operation modes in which a torque and a rotational speed are different. Under these different conditions, it is difficult for the conventional PM motor using the constant magnetic flux of a permanent magnet to keep the optimum state under all these conditions, and thus the efficiency of a system is reduced, noise is produced and other problems occur.
By contrast, since the variable magnetic flux drive system described above can vary the amount of magnetic flux of a permanent magnet, it can be expected that the efficiency is enhanced as compared with the conventional PM motor drive system with the fixed magnet. Furthermore, when the magnet is unnecessary, it is also possible to minimize the induced voltage by reducing the amount of magnetic flux.